Many ceramic articles of commerce are provided with smooth, glassy ceramic finishes commonly referred to as glazes. Merely by way of example, ceramic plumbing supplies such as bath tubs, sinks and toilets, commonly referred to in the ceramic trade as "white ware," have glaze over an underlying ceramic structure. The glaze on such articles is important for both aesthetic and functional reasons. Defects in the glaze, such as fissures or cracks can mar the appearance of the article and can also create sanitary difficulties, corrosion, and other functional problems in use. Thus, substantial defects in the glaze render the article unsaleable and require that the article either be scrapped or else reworked to repair the glaze.
Common rework processes involve application of a powdered material or frit having composition similar to the glaze layer to be repaired, followed by refiring of the entire article. In essence, the refiring process involves repetition of the firing process originally used to form the glaze layer, i.e. heating of the entire article to a temperature above the reflow temperature of the glaze layer, followed by slow, progressive cooling of the article. The cost of such a refiring step is a major portion of the cost of the original article. The ceramics industry therefore has long sought a more practical repair process.
Various attempts have been made to repair glaze defects with non-ceramic materials, such as paints or epoxies. These methods avoid the refiring step, but pose other difficulties with respect to chemical compatibility, color matching of the repair with the original glaze and durability of the repair. Brockway et al, U.S. Pat. No. 4,338,114 and Heineken et al, U.S. Pat. No. 4,731,254 both disclose use of a laser beam to melt a localized region of a glass article surrounding a crack, scratch or other defect, thereby causing local melting of the glass and repairing the article. Both of these patents explicitly require that the entire article be preheated to about the softening temperature before exposure to the laser beam. This mitigates the thermal stresses occurring during the localized heating by the laser and the subsequent cooling of the locally-heated region. The need for such preheating is a very substantial drawback. Thus, the preheating and the required slow cooling after preheating, would be essentially as expensive as the refiring process without the laser treatment.
Petitbon, U.S. Pat. No. 4,814,575 and the corresponding article by Petitbon et al, "Laser Surface Treatment of Ceramic Coatings", Materials Science and Engineering, A121, pp. 545-548 (1989) disclose processes for treating zirconia coatings such as those used on turbine blades and diesel engine parts. In these processes, a laser beam is swept across the surface while powdered alumina is dispensed in the beam. The powdered alumina melts, apparently along with some of the zirconia of the original article, thereby forming a combined phase at the surface. These references do not specify the bulk temperature of the base zirconia layer during the process or the particular configuration of the laser beam. The patent employs very rapid scanning, so that the laser typically interacts with any point on the surface for considerably less than 1 s, and typically less than 0.1 s. The ultimate conclusion as stated in the Petitbon article is that even with a relatively robust material such as zirconia, "random microcracking" occurs unless there is "reactive treatments with chemical modifications of the zirconia surface" as provided by the alumina powder. Manifestly, this work does not offer any suggestion that its techniques could be applied to ordinary ceramics such as those used in the whiteware industry, or, particularly, to the glazes applied to articles such as whiteware.
Attempts have been made heretofore to devise a scheme for localized reglazing of defective whiteware products by application of a laser beam, using theoretical predictions of the heat transfer and stresses induced by various beam conditions. These attempts have shown that any beam conditions which succeed in melting the glaze will also result in cracking of the article, i.e., that the process is impossible.
Thus, there are still very substantial, unmet needs for improved methods and apparatus to locally glaze the surfaces of ceramic articles and, particularly, to locally reglaze the surfaces of articles such as whiteware having glaze defects.